Inhibition characteristics of tri-peptides IKW and IKY, which were obtained as angiotensin-I converting enzyme inhibitors from bonito protein, and their stability in digestive canal were studied. These IKW and IKY showed high inhibitory activities with IC50 of 0.4 and 1.0 mu M, respectively, and acted as competitive inhibitors. The tri-peptides, especially IKY, were very stable under simulated gastrointestinal digestion conditions.